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LIGNITEŚCOAL

309

this exists in several varieties. Sometimes it has the aspect and colour
of wood (fossil wood) and sometimes it is brown, friable and easy to break
(peaty, earthy lignite) ; in some cases it consists of superposed layers (schistose
lignite) and in others is compact and varying in colour from brown to shining
black (pitch-'coal).

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When newly won, lignites contain 20-60 % of moisture, and when air-dried,
10-20%. The percentage of ash is very variable and, although it usually varies
;rom 2 to 15%, it may also be much greater. The elementary composition
referred to the fuel free from ash and moisture, generally varies between the
following limits :

Carbon
Hydrogen .
Oxygen
Nitrogen   .

55-75%

4-7%

20-35%

0-5-2%

In some lignites sulphur may be present in marked proportions.    The calorific
power of good lignites varies from 4,000 to 6,500 cals.

Table XXXVII (see p. 310) gives the analytical results for various lignites.

COAL

Coal proper includes the bituminous coals and anthracite ; there is a
gradual transition from the one to the other and no sharp delimitation.
They constitute the most important industrial fuels. They are usually
compact and black, the following types being distinguished: shining,
black coal; opaque, black; cannel, of a velvety, blackish colour, with a
conchoidal fracture ; fibrous coal; and bituminous slates (boghead).

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According to their chemical composition, coals are classified, after
Gruner, in six categories, which differ in the quantity and quality of the coke
they furnish and in their calorific powers. The normal limits for each of these
classes are indicated in Table XXXVIII (p. 311), the data in which are referred
to the pure fuel (free from moisture and ash). more.